Integrated circuits operate reliably if the control signals supplied to the integrated circuit always assume values within a set of limited values. In particular, the supply voltage Vcc should always have a value below a high threshold voltage. If this threshold voltage is exceeded, an uncontrolled operation of the integrated circuit may occur. For example, abnormal access to protected data of an integrated circuit may occur with an abnormally high value of supply voltage Vcc (beyond the upper limit of the set of limit values). As a result, a fraudulent individual could access a protected system and modify, for example, the information contained in a bank card.
Undesirable operations facilitating fraud are prevented by the use of a high threshold supply voltage detector integrated into the circuit. Such a detection circuit protects the system by enabling, for example, the locking of access to the system, or its memory, as soon as the detector detects a higher supply voltage than the detection threshold voltage value.
It is important for the detection threshold voltage value to remain constant. For example, integrated circuits can work in a very wide range of temperatures for example, a temperature range of -40.degree. C. to +85.degree. C.), so the detection threshold voltage value must remain substantially constant over this temperature range. Additionally, integrated circuits may vary substantially with different methods used to manufacture the circuit, so the value of the threshold voltage must remain substantially constant with each method of manufacture of the integrated circuit.
In known detectors, the detection threshold voltage varies greatly with the temperature of the circuit's environment and with the manufacturing methods used to fabricate the detector. In certain cases, it is possible to compensate for certain variations, as in the case of the detector described in the French patent application No. FR 89 16650 filed on behalf of the present Applicant. In this example, the detector is formed in a standard way by coupling a resistive voltage divider having an upper and lower arm to a CMOS inverter. The upper arm of the voltage divider, between Vcc and the voltage divider output node, comprises a P-channel transistor in series with a first N-channel transistor. The lower arm, between the output node and the ground, comprises a second N-channel transistor.
The N-channel and P-channel transistors in the upper arm are mounted as forward-biased diodes. In this structure, the first N-channel transistor has the function of compensating for the variations with the temperature of the N-channel transistor of the lower arm of the divider. The two N-channel transistors are technologically identical (with the same dimension and the same threshold voltage).
The known compensation techniques are not sufficient to keep the detection threshold voltage stable from one detector to the next due to technological vairations in the manufacturing methods and variations in temperature.
In manufacturing such a detector, an N-channel transistor has a typical threshold voltage at 0.degree. C. of 0.833 volts. In practice, the threshold voltages range from 0.697 (Vtn.sub.min) to 0.978 volt (Vtn.sub.max), giving an absolute difference of the order of 200 to 300 millivolts. A P-channel transistor has a typical threshold voltage at 0.degree. C. of -1.103 volts. In practice, threshold voltages range from -1.206 (Vtp.sub.min) to -1.016 volts (Vtp.sub.max), giving an absolute difference of the order of 200 millivolts.
With technological and temperature variations in the threshold voltages, the following values are thus obtained, in one example, for the detection threshold voltage value, Vo.
Vo equals 5.18 volts at 85.degree. C. for values of Vtn and Vtp of 0.697 and -1.206, respectively. PA1 Vo=8.85 volts at -40.degree. C. and for values of Vtn and Vtp of 0.978 and -1.016, respectively.
These results demonstrate a difference of Vo on the order of 3.5 volts for the temperature range considered (85.degree. C. to -40.degree. C.). The threshold voltage values Vtn and Vtp are all given at 0.degree. C.
In protected circuits, it is necessary to precisely keep signals within a set of limited values. The detectors currently known in the art cannot reliably or precisely keep voltage signals within a limited set of values with such a wide threshold voltage variation.